Edward Snowden, who has admitted leaking top-secret documents detailing the NSA's phone and Internet surveillance programs, exceeded his authorized access to the agency's computer systems while managing to smuggle out classified documents on a portable USB drive.

The small data-storage device has been banned from use on secret military networks, including those of the National Security Agency, for at least five years, The Washington Times reports.

"He should not have been able to do either of those things" without raising red flags, a private-sector IT security specialist told the Times.

NSA officials "were lying down on their job if they didn't disable the USB port," the specialist said, referring to the small socket on the side of a computer where the thumb-sized drives are inserted.

The agency is still trying to determine the extent of Snowden's breach of top-secret information. It did not respond to a request for comment from The Washington Times.

Snowden, a former NSA contractor, smuggled electronic copies of an unknown number of classified documents out of the agency's operations center in Hawaii, where he worked, The Los Angeles Times reported on Friday.

A U.S. official told The Washington Times "that's one avenue" investigators are following.

Thumb drives have been banned from use on classified military systems since malicious software, thought to be of Russian origin, infected the secret computer networks of the U.S. Central Command five years ago, The Washington Times reports.

Lawmakers briefed on the matter by NSA Director Gen. Keith Alexander have not commented on the USB drive, but said Snowden also exceeded his authorized access to the agency's computer systems.

"It's clear that he attempted to go places that he was not authorized to go, which should raise questions for everyone," Michigan Rep. Michael Rogers, the GOP chairman of the House Intelligence Committee, told the Times.